“Flower-like” Li4Ti5O12-multiwalled carbon nanotube composite structures with performance as highrate anode-materials for li-ion battery applications and methods of synthesis thereof

ABSTRACT

A method of fabricating nanocomposite anode material embodying a lithium titanate (LTO)-multi-walled carbon nanotube (MWNT) composite intended for use in a lithium-ion battery includes providing multi-walled carbon nanotube (MWNTs), including nanotube surfaces, onto which functional oxygenated carboxylic acid moieties are arranged, generating 3D flower-like, lithium titanate (LTO) microspheres, including thin nanosheets and anchoring the acid-functionalized MWNTs onto surfaces of the 3D LTO microspheres by π-π interaction strategy to realize the nanocomposite anode material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application derives the benefit of the filing date of U.S.Provisional Patent Application No. 62/618,248, filed Jan. 17, 2018. Thecontents of the provisional application are incorporated herein byreference.

GOVERNMENT SUPPORT STATEMENT

This invention was made with government support under DE-SC0012673awarded by the US Department of Energy. The government has certainrights in the invention.

BACKGROUND OF THE INVENTION

The invention broadly relates to lithium-ion batteries, and moreparticularly relates to a method for fabricating a nanocomposite anodematerial, comprising a lithium titanate (LTO)-multi-walled carbonnanotube (MWNT) composite structures, the method including providingacid functionalized multi-walled carbon nanotube (MWNTs), generating 3Dflower-like, lithium titanate (LTO) microspheres and anchoring theacid-functionalized MWNTs onto surfaces of the 3D LTO microspheres byπ-π interaction process to synthesize the nanocomposite anode material.

BACKGROUND OF THE RELATED ART

One of the key goals of sustainability is to create reliable, efficient,and cost-effective alternatives for fueling and powering conventionaldevices. To that end, the development of Li-ion batteries (LIB s)possessing high rate performance, superior durability and desirableenvironmental sustainability is critical to advancing applicationsranging from smart electronics to electric vehicles. C. F. Lin, X. Y.Fan, Y. L. Xin, F. Q. Cheng, M. O. Lai, H. H. Zhou, and L. Lu, J. Mater.Chem. A, 2, 9982 (2014); J. Guo, W. Zuo, Y. Cai, S. Chen, S. Zhang, andJ. Liu, J. Mater. Chem. A, 3, 4938 (2015); Y. Sha, B. Zhao, R. Ran, R.Cai, and Z. Shao, J. Mater. Chem. A, 1, 13233 (2013). Conventional LIBsassociated with graphite anode materials may suffer from lithiumdeposition on the anode surface, leading to poor cycling stability. G.N. Zhu, Y. G. Wang, and Y. Y. Xia, Energy Environ. Sci., 5, 6652 (2012);Y. F. Tang, L. Yang, S. H. Fang, and Z. Qiu, Electrochim. Acta, 54, 6244(2009); J. L. Qiao, Y. Y. Liu, F. Hong, and J. J. Zhang, Chem. Soc.Rev., 43, 631 (2014).

Consequently, spinel Li₄Ti₅O₁₂, also referred to as lithium titanate(LTO), has been proposed and investigated as an alternative anodematerial. LTO material has known advantages including: (i) a high andstable potential plateau value (i.e. 1.55 V versus Li/Li⁺). This avoidsthe formation of lithium dendrites and mitigates the formation of thesolid-electrolyte-interphase via electrolyte reduction. LTO materialdisplays (ii) excellent durability over an extended cycle life due to anegligible volume change during electrochemical cycling as well as (iii)a high thermal stability, potentially enabling their use at elevatedtemperatures. G. N. Zhu, Y. G. Wang, and Y. Y. Xia, Energy Environ.Sci., 5, 6652 (2012); J.-H. Choi, W.-H. Ryu, K. Park, J.-D. Jo, S.-M.Jo, D.-S. Lim, and I.-D. Kim, Sci. Rep., 4, 7334 (2014).

Known prior art methods for fabricating LTO material for anode use,however, are not found to overcome the low conductivity (<10⁻¹³ S cm⁻¹)inherent in utilizing bulk LTO material for same, or the inherentlymoderate lithium ion diffusion coefficient (10⁻⁹-10⁻¹³ cm² s⁻¹)associated therewith. G. N. Zhu, Y. G. Wang, and Y. Y. Xia, EnergyEnviron. Sci., 5, 6652 (2012); S. Kim, S. H. Fang, Z. X. Zhang, J. Z.Chen, L. Yang, J. E. Penner-Hahn, and A. Deb, J. Power Sources, 268, 294(2014); H. F. Ni and L. Z. Fan, J. Power Sources, 214, 195 (2012).

One known approach for ameliorating rate performance of LTO electrodesincluded designing unique nanostructure motifs of LTO in an attempt toenhance the respective composite structure's electronic and Li-ionconductivity. This was conducted within a context of zero-dimensional(OD) nanoparticles, J. Lim, E. Choi, V. Mathew, D. Kim, D. Ahn, J. Gim,S. H. Kang, and J. Kim, J. Electrochem. Soc., 158, A275 (2011); L. Sun,J. P. Wang, K. L. Jiang, and S. S. Fan, J. Power Sources, 248, 265(2014), one-dimensional (1D) nanowires and nanotubes, L. F. Shen, E.Uchaker, X. G. Zhang, and G. Z. Cao, Adv. Mater., 24, 6502 (2012); J.Liu, K. P. Song, P. A. van Aken, J. Maier, and Y. Yu, Nano Lett., 14,2597 (2014), and three-dimensional (3D) structural architectures. J. Z.Chen, L. Yang, S. H. Fang, and Y. F. Tang, Electrochim. Acta, 55, 6596(2010); D. Kong, W. Ren, Y. Luo, Y. Yang, and C. Cheng, J. Mater. Chem.A, 2, 20221 (2014). Relatedly, the inventor(s) successfully generated‘flower-like’ LTO microspheres, consisting of thin, saw-tooth shapedconstituent nanosheets synthesized by (i) a facile and large-scalehydrothermal process involving recyclable precursors followed by (ii) ashort, relatively low-temperature calcination process in air. The thinnanosheets gave rise to shortened Li-ion diffusion distances andenhanced contact area with electrolyte, while the micron-scale sphericalassemblies themselves possessed thermodynamic stability and high tapdensity. See, J. Z. Chen, L. Yang, S. H. Fang, and Y. F. Tang,Electrochim. Acta, 55, 6596 (2010); H. Xia, Z. T. Luo, and J. P. Xie,Nanotechnol. Rev., 3, 161 (2014). Also see, D. Kong, W. Ren, Y. Luo, Y.Yang, and C. Cheng, J. Mater. Chem. A, 2, 20221 (2014).

Electrodes made with the LTO microspheres were tested and found toexhibit excellent rate capabilities and stable cycling performance,delivering, as an example, as much as 137 mAh g⁻¹ with a capacityretention of about 87% at a discharge rate of 20 C from cycles 101 to300. It is well known in the field that a C rate is a measure of therate at which a battery is (dis)charged relative to its theoreticalcapacity, where the higher the C rate, the faster the rate of(dis)charge (i.e. 1C=1 hour rate, 5C=⅕=0.2 hour rate, C/20=20 hourrate). L. Wang, Y. M. Zhang, M. E. Scofield, S. Y. Yue, C. McBean, A. C.Marschilok, K. J. Takeuchi, E. S. Takeuchi, and S. S. Wong, ChemSusChem,8, 3304 (2015).

Another known approach to enhance the electronic conductivity betweenthe LTO anode material and the current collector (for Li-ion batteryuse) by mediating a conductive coating onto an underlying LTO surface,for example, of carbon nanotubes. H. F. Ni and L. Z. Fan, J. PowerSources, 214, 195 (2012); L. Shen, X. Zhang, E. Uchaker, C. Yuan, and G.Cao, Adv. Energy Mater., 2, 691 (2012); G. B. Xu, W. Li, L. W. Yang, X.L. Wei, J. W. Ding, J. X. Zhong, and P. K. Chu, J. Power Sources, 276,247 (2015).

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindricalnanostructure and are known to display exceptional electricalconductivity and outstanding mechanical strength in part due to theiranisotropic structure, and represent a promising conductive additive forimproving rate capabilities of LTO composites; J.-H. Choi, W.-H. Ryu, K.Park, J.-D. Jo, S.-M. Jo, D.-S. Lim, and I.-D. Kim, Sci. Rep., 4, 7334(2014); W. D. Zhang, B. Xu, and L. C. Jiang, J. Mater. Chem., 20, 6383(2010); A. Marschilok, C. Y. Lee, A. Subramanian, K. J. Takeuchi, and E.S. Takeuchi, Energy Environ. Sci., 4, 2943 (2011).

Research shows that CNTs can be incorporated within the context of OD,1D, and 2D LTO-based anode materials, respectively, using eitherphysical mixing or in situ deposition methods. J. J. Huang and Z. Y.Jiang, Electrochim. Acta, 53, 7756 (2008); Y. R. Jhan and J. G. Duh, J.Power Sources, 198, 294 (2012); X. L. Jia, Y. F. Kan, X. Zhu, G. Q.Ning, Y. F. Lu, and F. Wei, Nano Energy, 10, 344 (2014); H. K. Kim, K.C. Roh, K. Kang, and K. B. Kim, RSC Adv., 3, 14267 (2013); J. Shu, L.Hou, R. Ma, M. Shui, L. Y. Shao, D. J. Wang, Y. L. Ren, and W. D. Zheng,RSC Adv., 2, 10306 (2012); P. Zhang, M. Chen, X. Shen, Q. Wu, X. Zhang,L. Huan, and G. Diao, Electrochim. Acta, 204, 92 (2016). These methodsrely on physical contact between the LTO and CNT, which is typically aweak attachment mode.

Representative examples of these varied efforts are now presented. Forexample, Fang, et al., prepared LTO/CNT-based composites by physicallymixing, by ball milling, to embed submicron LTO particles within anetwork of conductive MWNTs. The resulting composite was delivered ˜100mAh/g at a 30C discharge rate. W. Fang, P. J. Zuo, Y. L. Ma, X. Q.Cheng, L. X. Liao, and G. P. Yin, Electrochim. Acta, 94, 294 (2013).

Ni, et al. reported on the use of CNTs to which LTO nanoparticles hadbeen immobilized by liquid phase deposition as a composite anodematerial for high rate LIBs. These materials delivered ˜90 mAh/g at a30C discharge rate.

Shen, et al., were grew LTO sheathes with a measured thickness ofapproximately 25 nm on the exterior of a MWNT core which delivered ˜90mAh/g at a 40C discharge rate. L. F. Shen, C. Z. Yuan, H. J. Luo, X. G.Zhang, K. Xu, and F. Zhang, J. Mater. Chem., 21, 761 (2011).

Zhang et al. recently fabricated LTO nanosheet/CNT composites, yielding145 mAh g¹ and 118 mAh g⁻¹ at discharge rates of 11 C and 23 C,respectively. P. Zhang, M. Chen, X. Shen, Q. Wu, X. Zhang, L. Huan, andG. Diao, Electrochim. Acta, 204, 92 (2016).

Considering the studies described above, better rate performance isneeded for high performance battery and cell anodes. Therefore, animproved method of attaching the LTO to the CNT is desired.

SUMMARY OF THE INVENTION

The invention provides a method for synthesizing a nanocomposite anodematerial for lithium ion batteries that overcomes the shortcomings ofthe prior art.

The invention comprises a process (method) for synthesizing compositestructures, i.e. “motifs”, formed of a “flower-like” Li₄Ti₅O₁₂ (lithiumtitanate or LTO) multiwalled carbon nanotubes (MWNTs) for use ashigh-rate anode materials for Li-ion battery applications. The inventionfurther includes an anode material fabricated by the process (method)and a lithium-ion battery formed with same.

The anode material fabricated by the inventive process comprises a 3Dhierarchical flower-like lithium titanate-multiwalled carbon nanotubes(LTO-MWNT) composite structures, which allows for precise tuning andoptimizing the nature of the interactions between of 3D LTO microsphereswith the underlying MWNTs, within an MWNT framework.

As will be explained in greater detail herein, nanoscale attachmentmodality is a significant as it is an important determinant of observedelectrochemical performance. Specifically, the invention relies uponcontrolled loading ratios of multi-walled carbon nanotubes (MWNTs) tosuccessfully anchor onto the surfaces of a unique “flower-like”Li₄Ti₅O₁₂ (LTO) microscale sphere structure using a number of differentand distinctive preparative approaches, including (i) a sonicationmethod, (ii) an in situ direct-deposition approach, (iii) a covalentattachment protocol, and (iv) a π-π interaction strategy.

As known, a hybrid composite possesses a number of ‘structural design’advantages that can assist in improving a measured anode performance. Tofabricate the 3D hierarchical flower-like lithium titanate-multiwalledcarbon nanotube (LTO-MWNT) composite structure, thin constituentnanosheets within the flower-like LTO micron-scale spheres are firstprovided for a reduced lithium ion diffusion distance. N. Li, T. Mei, Y.Zhu, L. Wang, J. Liang, X. Zhang, Y. Qian, and K. Tang, Cryst Eng-Comm,14, 6435 (2012); Y. Tang, F. Huang, W. Zhao, Z. Liu, and D. Wan, J.Mater. Chem., 22, 11257 (2012).

The numerous roughened surfaces of the thin, petal-like nanosheets,associated with a high surface area, represent potentially favorableactive sites for the interaction of the electrolyte with LTO, therebyultimately providing for improved voltage profiles and charge/dischargedynamics. Then, the MWNTs or MWNT network provide for viable electricalpathways to the LTO flower-like structure from the current collector,and increase the mechanical stability of the underlying electrodethrough the interweaving of the electrode components. J.-H. Choi, W.-H.Ryu, K. Park, J.-D. Jo, S.-M. Jo, D.-S. Lim, and I.-D. Kim, Sci. Rep.,4, 7334 (2014).

While prior art efforts to generate MWNT-LTO nanocomposites to enhanceelectrode performance, few if any efforts are known that primarily focuson chemically controlling and improving upon the ion transport betweenthe constituent LTO motifs and the adjoining CNTs, i.e., systematicallyengineering the nature of the molecular junction between these twospecies through judiciously chosen attachment strategies.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the description of embodiments that follows, with reference to theattached figures, in which:

FIG. 1 is a reaction schematic denoting the covalent attachment processbetween “flower shape” LTO micron-scale spheres and adjoining MWNTs;

FIG. 2 is a reaction schematic demonstrating the π-π interactionsbetween the “flower shape” LTO micron-scale spheres and adjoining MWNTs;

FIG. 3 depicts, top to bottom, XRD patterns associated with (i) theoxidized MWNTs, (ii) pure flower shape LTO motifs, and (iii) theresulting LTO-MWNT composites, generated by sonication, direct in situ,covalent attachment and π-π interaction strategies, respectively;

FIG. 4A depicts a TEM image of the LTO-MWNT composite generated bysonication;

FIG. 4B depicts an SEM TEM image of the LTO-MWNT composite generated bysonication;

FIG. 4C depicts a TEM image of the LTO-MWNT composite generated by insitu attachment;

FIG. 4D depicts an SEM TEM image of the LTO-MWNT composite generated byin situ attachment;

FIG. 4E depicts a TEM image of the LTO-MWNT composite generated bycovalent attachment;

FIG. 4F depicts an SEM TEM image of the LTO-MWNT composite generated bycovalent attachment;

FIG. 4G depicts a TEM image of the LTO-MWNT composite generated by π-πinteraction synthesis process;

FIG. 4H depicts an SEM TEM image of the LTO-MWNT composite generated byinteraction synthesis process;

FIG. 5A is an HRTM image and the corresponding insets to the SAEDpatterns of LTO-MWNT composites generated by A) physical sonication;

FIG. 5B is an HRTM image and the corresponding insets to the SAEDpatterns of LTO-MWNT composites generated by B) in situ growth;

FIG. 5C is an HRTM image and the corresponding insets to the SAEDpatterns of LTO-MWNT composites generated by C) covalent attachment;

FIG. 5D is an HRTM image and the corresponding insets to the SAEDpatterns of LTO-MWNT composites generated by D) π-π interactionstrategies;

FIG. 6 illustrates IR spectra of (a) oxidized MWNTs, (b) pure LTO, (c)APTES linkers, as well as (d) 4-MBA-functionalized LTO motifs, andassociated LTO-MWNT heterostructures, generated not only by a (e)covalent attachment but also (f) π-π interaction strategies;

FIG. 7a depicts a voltammograms of the 5% in situ sample, revealingreversible electrochemistry at all scan rates wherein clear anodic andcathodic peaks were apparent;

FIG. 7b depicts the 5% physically sonicated LTO-MWNT sample demonstratedA-Epeak values of 0.36, 0.44, 0.56, and 0.70 Vat scan rates of 0.5, 1.0,2.0, and 5.0 mV/s, respectively;

FIG. 7C depicts A-Epeak values of 0.32, 0.44, 0.62, and 0.93 V at scanrates of 0.5, 1.0, 2.0, and 5.0 mV/s, respectively, for the 5% covalentLTO-MWNT sample;

FIG. 7d depicts A-Epeak values of 0.25, 0.35, 0.45, and 0.61 Vat scanrates of 0.5, 1.0, 2.0, and 5.0 mV/s, respectively, for the LTO-MWNTcomposite prepared through π-π interactions;

FIG. 8a presents fits of the cyclic voltammetry (CV) data to theRandles-Sevcik equation, plots of i_(p) as a function of square root ofthe scan rate for cathodic peaks;

FIG. 8b presents fits of the CV data to the Randles-Sevcik equation,plots of i_(p) as a function of square root of the scan rate for anodicpeaks;

FIG. 9a depicts discharge and charge voltage curves for Lithium/LTO-MWNTelectrochemical cells, at cycle 20 (C/2 rate), 25 (20 C rate), 30 (50 Crate), 35 (100 C rate), comprising active material generated using a 5%MWNT loading amount with in situ method;

FIG. 9b depicts discharge and charge voltage curves for Lithium/LTO-MWNTelectrochemical cells at cycle 20 (C/2 rate), 25 (20 C rate), 30 (50 Crate), 35 (100 C rate), comprising active material fabricated using a 5%MWNT loading amount with the physical sonication protocol;

FIG. 9c depicts discharge and charge voltage curves for Lithium/LTO-MWNTelectrochemical cells at cycle 20 (C/2 rate), 25 (20 C rate), 30 (50 Crate), 35 (100 C rate), comprising active material produced using a 5%MWNT loading amount with covalent attachment;

FIG. 9d depicts discharge and charge voltage curves for Lithium/LTO-MWNTelectrochemical cells at cycle 20 (C/2 rate), 25 (20 C rate), 30 (50 Crate), 35 (100 C rate), comprising active material generated using a 5%MWNT loading amount synthesized through π-π interactions;

FIG. 10 depicts discharge capacity versus cycle number forlithium/LTO-MWNT electrochemical cells created with active materialcomposites prepared using sonication (black triangles), covalentattachment (black squares), in situ deposition (black circles) and π-πinteractions (blue triangles), respectively at a 5% MWNT loadinglevel—the 3D flower LTO “control” sample (red squares) tested in thesame program also is shown to effect comparison; and

FIG. 11 depicts an equivalent circuit (top) and electrochemicalimpedance spectroscopy data (bottom) for lithium/LTO-MWNTelectrochemical cells, incorporating active material compositesfabricated using physical sonication (black triangles), covalentattachment (squares), in situ deposition (circles) and π-π interaction(blue triangle) protocols, respectively, at a 5% MWNT loading level.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of embodiments of the invention willbe made in reference to the accompanying drawings. In describing theinvention, explanation about related functions or constructions known inthe art are omitted for the sake of clarity in understanding the conceptof the invention to avoid obscuring the invention with unnecessarydetail.

In separate experiments, the inventors embedded 3D LTO micro spheresinto the MWTNs (i.e., MWNT networks) through (i) physical sonication,(ii) direct in situ deposition, (iii) covalent attachment, and (iv)simple, π-π interaction strategies.

It should be noted that incorporation of various linkers as connective,mediating ‘bridges’ between LTO and MWNTs with the goal of enhancingelectrode performance for methods (iii) and (iv) are heretobeforeunknown with prior LTO-MWNT composite formation. The above-mentionedexperiments sought to correlate the electrochemical performances ofthese individual distinctive LTO-MWNT composites with their specificattachment strategy, i.e. with the particular preparative treatmentprocess used to generate each of these composite materials.

Inventive composites generated by physical sonication as well as π-πinteractions retain the intrinsic hierarchical “flower-like” morphologyand exhibit a similar crystallinity profile as compared with that ofpure LTO.

Inventive composites prepared by in situ direct deposition yielded afragmented LTO structure (likely due to the possible interferingpresence of the MWNTs themselves during the relevant hydrothermalreaction) and a larger crystallite size, owing to the higher annealingtemperature associated with its preparation.

Inventive composites created via covalent attachment was covered with anamorphous insulating linker, which probably led to a decreased contactarea between the LTO and the MWNTs and hence, a lower crystallinity inthe resulting composite.

Inventive composites generated by π-π interactions out-performed theother three analogous heterostructures, i.e., those formed by physicalsonication, in situ direct deposition and covalent attachment, werefound (through electrode testing), due to a smaller charge transferresistance as well as a faster Li-ion diffusion. The inventive LTO-MWNTcomposite (material), produced by π-π interactions, exhibits areproducibly high rate capability as well as a reliably solid cyclingstability, delivering 132 rnAh g⁻¹ at 50 C, after 100 discharge/chargecycles, including 40 cycles at a high (>20 C) rate. Such data denote thehighest electrochemical performance measured to date as compared withany LTO-carbon nanotube-based composite materials previously reported,under high discharge rate conditions. The data tangibly underscore thecorrelation between preparative methodology and the resultingperformance metrics.

The experimental testing verified successful synthesis of the“flower-like” LTO-MWNT composites systematically fabricated viadifferent preparative approaches, including (i) physical sonication,(ii) an in situ direct deposition approach, (iii) a covalent chemicalattachment protocol, as well as (iv) a π-π interaction strategy. Dataderived from a structural characterization analysis suggest that thecomposites generated by both the physical sonication and π-π interactionmethods retain not only the favorably small crystallite size (i.e. 12.6nm) of the pure LTO but also the intrinsic “flower-like” morphologyascribed to pure LTO, both during and after the preparative process.

By contrast, the data show that composite prepared through an in situdirect-deposition approach yields fragmented LTO structures, possiblydue to curtailed crystal growth induced by the presence of interferingMWNTs, but also larger overall crystallite sizes likely resulting fromthe higher annealing temperatures used, denoting factors whichcontributed to the lower electrochemical performance measured. Rietveldrefinement results suggest the existence of the rutile and anatase TiO₂impurities within the composite, which were detected in the CV peaks.Rietveld refinement is a method well known in the field for phasecomposition analysis and determining crystal structure refinements basedon powder diffraction data. Finally, the composite produced via covalentattachment appeared to be enveloped with a coating of amorphous linkermolecules, thereby leading to not only lower overall crystallinity butalso a decreased contact area between LTO and MWNTs, thereby giving riseto poorer performance.

The electrochemical performance of these composite materials wascorrelated with their corresponding attachment chemistry. For example,an LTO sample with the 5% MWNT loading prepared via the π-π interactionmethod evidenced the highest delivered discharge capacity at every Crate. It is well known in the field that a C-rate is a measure of therate at which a battery is (dis)charged relative to its theoreticalcapacity, where the higher the C rate, the faster the rate of(dis)charge (i.e. 1C=1 hour rate=60 minute discharge, 5C=⅕=0.2 hourrate=12 minute discharge, C/20=20 hour rate=120 minute discharge) fromC/2 to 100C, with the most notable differences apparent under dischargerates ≥20 C, due to a much lower charge transfer resistance as comparedwith those of the other LTO-MWNT composites analyzed. These LTO-MWNTcomposites, produced by π-π interactions, exhibited a reproducibly highrate capability and a desirable cycling stability, i.e., delivering 174mAh g-1 at C/2 with a 99% capacity retention from cycles 20-90, 163 mAhg-1 at 20 C with a 97% capacity retention from cycles 25-95, and 146mAh/g at 50 C with a 90% capacity retention from cycles 30-100.

These observed parameters denote clearly superior performance to thoseof any previously reported LTO-carbon nanotube composite materials, todate, especially under these relatively low loading conditions. Notably,the LTO-MWNT samples prepared via the covalent attachment schemedelivered a lower capacity and displayed 97% capacity retention fromcycle 20 to cycle 90 at C/2 rate as compared with the higher capacityand 99% capacity retention for the set of physically sonicated, in situ,and π-π interaction samples. The voltammetric and galvanostatic datacoupled with the impedance results indicate slower kinetics for theLTO-MWNT heterostructures, prepared using the covalent attachmentapproach, suggests that increased charge transfer resistance was foundto have been associated with a covalent coupling protocol involving the3-aminopropyltriethoxysilane (APTES) linker.

Examples

Functionalization of MWNTs—

Pristine MWNTs (SES Research, 95% nanotubes and 2% amorphous carbons)were initially dispersed in concentrated HNO3 (Sigma-Aldrich, 70%) bysonication and further refluxing at 120° C. for 4 h in order to (a)remove any remnant metal catalysts and carbonaceous impurities, as wellas to (b) generate functional, oxygenated carboxylic acid moieties ontothe nanotube surfaces. The resulting purified and oxidized MWNTs,possessing a diameter range of 10 to 30 nm, were filtered through a 200nm pore diameter polycarbonate membrane (Millipore), thoroughly washedwith excess water, and dried at 80° C. for 18 h.

Synthesis of ‘Flower-Like’ LTO Micron-Scale Spheres—

Approximately 40 pieces of Ti foil (STREM chemicals, 99.7%), includingof 1 cm×1 cm squares, were placed in a 120 mL autoclave and reacted with86.1 mL of 0.5 M LiOH (Acros Organics™, 98%) and 7.83 mL of 30% (w/w)H₂O₂ (VWR) aqueous solution, followed by strong stirring at roomtemperature for 15 min. Afterwards, the as-prepared mixture solution wassubsequently heated at 130° C. for 4 h. The resulting suspension andas-formed white precipitate were separated by vacuum filtration, washedwith aliquots of deionized water, and ultimately oven dried at 80° C.The final products were annealed at 500° C. in air for 3 h in a mufflefurnace in order to obtain ‘flower-like’ LTO microspheres. As describedby L. Wang, Y. M. Zhang, M. E. Scofield, S. Y. Yue, C. McBean, A. C.Marschilok, K. J. Takeuchi, E. S. Takeuchi, and S. S. Wong, ChemSusChem,8, 3304 (2015).

Synthesis of ‘Flower-Like’ Li₄Ti₅O₁₂ Micron-Scale Spheres—MWNT CompositeStructures—

To demonstrate the chemistry of heterostructure formation, and electrodeperformance, composite structures were generated by four complementarystrategies, namely by means of (1) a physical mixing of preformedstructures by sonication; (2) a direct in situ deposition of MWNTs ontothe underlying LTO micron-scale spheres within the context of ahydrothermal reaction environment; (3) the covalent attachment of thetwo constituent components through the mediation of a silane linkermolecule; and (4) the formation of π-π interactions stabilized withshort-chain, electro active aromatic linker molecules.

Sonication Method.

The acid-functionalized MWNTs were well dispersed in dimethyl sulfoxide(DMSO) by ultrasonication for 1 h in order to obtain a clear blacksolution. As-prepared LTO micron-scale spheres were sonicated in waterfor 30 min before being added into the MWNT solution in a drop-wisemanner. The mixture solution was further ultra-sonicated for another 1h. The final product was collected by filtration, washed with deionizedwater, and dried at 80° C. in order to obtain the resulting LTO-MWNTcomposites.

In Situ Direct Deposition Approach.

The functionalized MWNTs were added to the autoclave together with theprecursors of lithium titanate, namely H₂O₂, LiOH, and Ti foil, usingthe same reaction parameters, as previously described. The resultinggrey product was further annealed at 600° C. for 3 h under an N₂atmosphere in a tube furnace in order to preserve the underlyingstructural integrity of MWNTs. L. Wang, Y. M. Zhang, M. E. Scofield, S.Y. Yue, C. McBean, A. C. Marschilok, K. J. Takeuchi, E. S. Takeuchi, andS. S. Wong, ChemSusChem, 8, 3304 (2015).

Covalent Attachment Protocol—

As-prepared LTO micron-scale spheres were initially functionalized with(3-aminopropyl) triethoxysilane (APTES) linker molecules (AcrosOrganics, 99%) by dispersing them in anhydrous organic DMSO solvent, inorder to inhibit the formation of undesirable polysilsesquioxane thatnormally is generated through the hydrolytic condensation oforganosilanes in either water or ethanol-water media. S. Sankaraiah, J.M. Lee, J. H. Kim, and S. W. Choi, Macromolecules, 41, 6195 (2008); I.Noda, T. Kamoto, and M. Yamada, Chem. Mater., 12, 1708 (2000); E.Asenath-Smith and W. Chen, Langmuir, 24, 12405 (2008). The coated LTOsample was subsequently reacted at 85° C. for 18 h under a N2atmosphere, followed by thermal curing at 120° C. for 24 h in order togenerate amine-terminated LTO.

The resulting NH₂-terminated LTO product was collected by centrifugationand further washed with DMSO for three times to remove any remaining,free-standing APTES molecules. The acid-functionalized MWNTs weredispersed in a H₂O: DMSO mixture (i.e., a 1:2 ratio by volume) bysonication followed by the addition of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) (Sigma-Aldrich, 99.9%), and N-hydroxysuccinimide(NHS) (Sigma-Aldrich, 98%) in order to activate pendant carboxylic acidgroups. The APTES-derivatized LTO were dispersed in water and mixed inwith the MWNT solution in 2-morpholino-ethanesulfonic acid (MES)(Sigma-Aldrich, 99.5%) buffer solution. The resulting mixture wasreacted under vigorous stirring for 24 h. The final product wasultimately filtered, washed with excess deionized water, and finallydried at 80° C. to obtain LTO-MWNT composites. FIG. 1 is a reactionschematic denoting the covalent attachment process between “flowershape” LTO micron-scale spheres and adjoining MWNTS.

π-π Interaction Strategy—

As-synthesized LTO spheres were initially dispersed in an ethanolicsolution of 4-mercaptobenzoic acid (4-MBA) (Aldrich, 99%) linkermolecules. The mixture was stirred at 60° C. for 18 h in order to createa sample of 4-MBA functionalized LTO, in which the terminal carboxylicacid groups of the ligand are bound onto the Ti sites localized on theLTO surface through either a mono dentate or bidentate coordinationmode. A. Raman, R. Quinones, L. Barriger, R. Eastman, A. Parsi, and E.S. Gawalt, Langmuir, 26, 1747 (2010).

The molecular-coated LTO product was isolated by vacuum filtration andfurther washed with ethanol for multiple times to remove any remaining,unbound 4-MBA linkers. Such 4-MBA functionalized LTO micro spheres weresubsequently reacted with the oxidized MWNTs, through sonication in amixture of ethanol and DMSO (in a 3:1 volume ratio) solvents for 2 h, inorder to facilitate favorable and stabilizing π-π interactions betweenthe phenyl rings within the aromatic, conjugated linker molecules andthe underlying MWNT network. The resulting composites were subsequentlyvacuum filtered, washed, and ultimately oven dried at 80° C. FIG. 2 is areaction schematic demonstrating the π-π interactions between the“flower shape” LTO micron-scale spheres and adjoining MWNTS.

In terms of structural-characterization, the composites generated byphysical sonication as well as π-π interactions retained the intrinsichierarchical “flower-like” morphology and exhibited a similarcrystallinity profile as compared with that of pure LTO. By comparison,the composite prepared by an in situ direct deposition approach yieldednot only a fragmented LTO structure, likely due to the possibleinterfering presence of the MWNTs themselves during the relevanthydrothermal reaction, but also a larger crystallite size, owing to thehigher annealing temperature associated with its preparation. Finally,the composite created via covalent attachment was covered with anamorphous insulating linker, which probably led to a decreased contactarea between the LTO and the MWNTs and hence, a lower crystallinity inthe resulting composite.

Electrode tests suggested that the composite generated by π-πinteractions out-performed the other three analogous heterostructures,due to a smaller charge transfer resistance as well as a faster Li-iondiffusion. In particular, the LTO-MWNT composite, produced by π-πinteractions, exhibited a reproducibly high rate capability as well as areliably solid cycling stability, delivering 132 rnAh g⁻¹ at 50 C, after100 discharge/charge cycles, including 40 cycles at a high (>20 C) rate.Such data denote the highest electrochemical performance measured todate as compared with any LTO-carbon nanotube-based composite materialspreviously reported, under high discharge rate conditions, and tangiblyunderscore the correlation between preparative methodology and theresulting performance metrics.

Morphology and Structure of the Materials—

Typical XRD patterns of (i) MWNTs, (ii) pure “flower-like” LTOmicron-scale spheres, and (iii) LTO-MWNT composites, prepared usingdifferent synthetic strategies, respectively, as depicted in FIG. 3. TheMWNT loading ratio in each composite was confirmed as 5 wt %. The pureLTO can be indexed to the face-centered cubic (fcc) spinel structure ofLi₄Ti₅O₁₂, where the broadened peaks in the XRD pattern suggest assynthesized LTO possesses a relatively small crystallite size, which iscalculated to be about 12.6 nm using the Debye-Scherrer equation. UponMWNT addition, the diffraction peak positions of the LTO did not change,with the major diffraction peaks located at 20 values of 18.4°, 35.7°,43.3°, 47.4°, 57.3°, 62.8°, and 66.2°, which could be ascribed to the(111), (311), (400), (331), (333), (440), and (531) planes,respectively, of a face-centered cubic spinel structure, possessing aFd-3m space group. A new and reasonably broadened peak centered at 26.2°could be assigned to the (002) lattice plane of the MWNTs, present inthe XRD profile of the MWNTs alone, as highlighted in the top panel ofFIG. 3. H.-K. Kim, K. C. Roh, K. Kang, and K.-B. Kim, RSC Adv., 3, 14267(2013).

When comparing these data with pure LTO, the LTO-MWNT formed by both thephysical sonication and π-π interaction methods yielded little if anyobvious change in the degree of overall crystallinity. In contrast, theLTO-MWNT sample prepared by the in situ deposition protocol, evincedsharper diffraction peaks suggestive of a larger crystallite sizepossibly due to higher annealing temperatures.

In effect, based upon the Debye-Scherrer equation, the crystallite sizederived from the composite prepared by the in situ method was calculatedto be 30.0 nm as compared with a 12.6 nm value associated with both pureLTO as well as with the two composites generated by physical sonicationand π-π interaction methods. It has been reported that the presence of alarger crystallite size may lead to a reduced rate performance of thiscomposite material as compared with analogous composites possessing asmaller crystallite size. This assertion has been ascribed to theobservation that a relatively small crystallite size often favorsenhanced rate capabilities and long-term cyclability, because of acombination of not only low charge transfer impedance but also decreasedLi⁺ diffusion impedance. G. Hasegawa, K. Kanamori, T. Kiyomura, H.Kurata, K. Nakanishi, and T. Abe, Adv. Energy Mater., 5, 1400730 (2015);Y. B. Shen, J. R. Eltzholtz, and B. B. Iversen, Chem. Mater., 25, 5023(2013).

The LTO-MWNT composite prepared by covalent immobilization gave rise toless well-defined XRD peaks and lower signal-to-noise ratios, reflectinga lower degree of crystallinity, which is believed to be attributable tothe presence of amorphous APTES linker molecules on the LTO surface.H.-Y. Cheng, L.-J. Lai, and F.-H. Ko, Int. J. Nanomedicine, 7, 2967(2012). The calculated crystallite size for this covalently formedcomposite was estimated to be 12.0 nm, similar to that of pure LTOalone, thereby indicating that the APTES functionalization processitself had not altered the apparent crystallinity of LTO itself.

To gain a deeper structural understanding of the various samples formed,Rietveld refinements were conducted and variations in the measuredlattice parameter, a, were observed. For example, pure LTO possesses alattice parameter of 8.35 Å, whereas the LTO-MWNT (5 wt %) compositesgenerated by sonication, direct in situ, covalent attachment, and π-πinteraction strategies yielded corresponding lattice parameters of 8.35Å, 8.36 Å, 8.36 Å, and 8.36 Å, respectively. The composite derived fromthe direct in situ method in particular contained 0.9% rutile TiO2 and0.8% anatase TiO2 impurities.

Our pure LTO micron-scale spheres exhibited a hierarchical“flower-shape” structure with an overall diameter of 1 μm. The thinconstituent, petallike component nanosheets measured 12.5±2.6 nm inthickness from a TEM image. The morphologies and micron-scale structuresof the LTO-MWNT (5 wt %) composites were investigated by TEM and SEM, aspresented in FIG. 4. The MWNTs, indicated by arrows, measure 10-30 nm indiameter and are intermingled with the LTO spheres.

The composite generated through a physical sonication method (FIGS. 4Aand 4B) evinced the presence of a relatively uniform coverage of evenlydistributed MWNTs with no observable morphological change associatedwith the underlying LTO micron-scale spheres. As for the correspondingcomposite synthesized by the direct in situ deposition technique, thepresence of the MWNTs might have potentially interfered with the growthof the LTO micron-scale spheres themselves during the process of thehydrothermal reaction, since we observed a certain degree offragmentation, thereby resulting in the formation of ‘broken up’,particulate LTO structures, possessing an average diameter of 35.2±5.3nm, as well as individual, dissociated nanosheets, C. F. Lin, X. Y. Fan,Y. L. Xin, F. Q. Cheng, M. O. Lai, H. H. Zhou, and L. Lu, J. Mater.Chem. A, 2, 9982 (2014), as indicated by the white circles in FIGS. 4Cand 4D.

By contrast, the inventors noted that the MWNTs appeared to be moreuniformly dispersed and distributed throughout the network of LTOmicron-scale spheres within the framework of composites generatedthrough a covalent attachment strategy (FIGS. 4E and 4F). However, theoccurrence of individual nanosheets themselves became less distinctive,possibly due to the presence of surface capping associated with APTESlinker molecules, as indicated by the yellow circles in FIG. 4F. Byanalogy with the LTO-MWNT composite derived from physical sonication,the composite prepared by π-π interactions (FIGS. 4G and 4H) alsoevinced a uniform MWNT coverage on the surface with little if anynoticeable morphological alteration of the LTO motif.

More detailed structural information was provided by HRTEM images andselected area electron diffraction (SAED) patterns, as shown in FIGS.5A-5D. In effect, within the HRTEM data, distinctive lattice fringespossessing distances of approximately 4.84 angstroms and 3.41 angstromswere observed, corresponding to the (111) planes of spinel LTO and theinterlayer spacings of graphitic layers within the MWNTs themselves,respectively. The corresponding SAED patterns could be indexed to the(111), (311), and (400) lattice planes of spinel LTO as well as the(002) lattice plane of MWNTs, respectively.

These data evidence a presence of phase purity, where the MWNTs are inclose contact with LTO nanosheets, providing for enhanced transportpathways between adjacent LTO micron-scale spheres. For that matter,FIG. 5C shows the covalently attached composite was less crystallinethan for the other 3 as-formed heterostructures (FIGS. 5A, 5B and 5D),an observation that was consistent with the XRD results.

In order to visualize and further reveal structural information at thejunctions between the MWNTs and the LTO microspheres, EDS mapping dataof the composites variously prepared by the four different attachmentmodalities studied. The C mapping signals are relatively diffuse in allfour samples, due to the presence of a carbon supporting film on the CuTEM grid. Nonetheless, it is expected that MWNTs and LTO microspheresare in direct contact with each other in samples prepared by bothsonication and in situ deposition methods, and not surprisingly, onlyTi, O, and C signals can be detected. By contrast, in the sampleprepared by the covalent attachment strategy, additional N and Sisignals likely stemming from the APTES linker were observed. Thesesignals spatially overlap rather well with the corresponding Ti and Opeaks associated with the LTO microspheres, thereby implying arelatively uniform coating of APTES molecules onto the underlying LTOsurface. Similarly, mapping results of the S element emanating from the4-MBA linker coincide closely with those of Ti and O within the sampleprepared by π-π interactions. These data likewise unambiguously confirmthe expected presence of 4-MBA molecules at the junctions between theLTO and the MWNT, a finding consistent with the sample preparationconditions.

FT-IR spectroscopy was used to confirm the formation of amide bonds aswell as π bonds within the LTO-MWNT composites formed by means ofcovalent attachment and π-π interactions, respectively. Specifically,spectra of (i) oxidized MWNTs, (ii) pure LTO, (iii) APTES, as well as(iv) 4-MBA-functionalized LTO, together with (v) LTO-MWNT 5 wt %composites, generated by both covalent attachment and π-π interactionmethods, are respectively shown in FIGS. 6a-6f . Upon treatment withnitric acid, the peaks located at 1720 cm-1 (FIG. 6 a) could be ascribedto the stretching bands of the C═O functionality derived from thecarboxylic acid group, Y. Si and E. T. Samulski, Nano Lett., 8, 1679(2008), thereby confirming the success of the acid functionalizationprotocol. It was noted that no distinctive peaks appeared for the pureLTO sample (FIG. 6b ). In the APTES-LTO species (FIG. 6c ), thecharacteristic peaks located at 1140 and 1030 cm−1 could be assigned tothe Si—OH and Si—O—Si groups associated with the APTES linker, R.Villalonga, M. L. Villalonga, P. Diez, and J. M. Pingarron, J. Mater.Chem., 21, 12858 (2011), whereas the peaks localized at 3430 cm-1 and1650 cm-1 were consistent with the N—H stretching and bending modesderived from the amine group, respectively. These data were indicativeof the likely successful functionalization involving APTES.

Peaks situated at 1650 cm-1, 3406 cm-1, and 1577 cm-1 and associatedwithin the LTO-MWNT composite derived from the covalent attachmentmethod (FIG. 6e ) likely corresponded to the C═O stretching bands inaddition to the N—H stretching and bending modes from the amide group,respectively, all of which were suggestive of the probable formation ofan amide bond between the MWNTs and the LTO micron-scale spheres. The4-MBA coated LTO sample (FIG. 6d ) gave rise to a sharp peak located at1680 cm-1, corresponding to the stretching mode of the C═O bondassociated with the 4-MBA linker. Peaks located at 1591 and 1425 cm-1could be ascribed to the stretching mode of the phenyl ring derived fromthe linker, an observation consistent with a successfulfunctionalization process. After attaching the MBA-coated LTO onto theMWNTs (FIG. 60, the ring stretching peak shifted from 1425 cm-1 to 1398cm-1, a result which was likely induced mainly by strong π-π stackinginteractions between the phenyl ring in the MBA linker and theconjugated MWNT network, and consequently, a “softening” of the C═Cbonds, i.e. indicative of an expansion of C—C bonds. D.-Q. Yang, J.-F.Rochette, and E. Sacher, J. Phys. Chem. B, 109, 4481 (2005); Y. Zhang,S. Yuan, W. Zhou, J. Xu, and Y. Li, J. Nanosci. Nanotechnol., 7, 2366(2007).

Electrochemical Properties of LTO-MWNT Composite Heterostructures:Cyclic Voltammetry—

Cyclic voltammetry (CV) of two electrode cells containing the LTO-MWNTheterostructures was collected at scan rates of 0.5, 1.0, 2.0, and 5.0mV/s, as shown in FIGS. 7a, 7b, 7c and 7d . The voltammograms of the 5%in situ sample revealed reversible electrochemistry at all scan rateswherein clear anodic and cathodic peaks were apparent, as indicated inFIG. 7a . The A-Epeak values were 0.36, 0.36, 0.48, and 0.60 V at thescan rates of 0.5, 1.0, 2.0, and 5.0 mV/s, respectively. Under theslower scan rates of 0.5 and 1.0 mV/sec, two features were present forthe anodic wave.

In data associated with the ‘in situ’ prepared sample, the reductionpeak noted at 1.4 V can be attributed to the irreversible formation ofthe LiTiO₂ phase from the rutile TiO₂ phase, D. Wang, D. Choi, Z. Yang,V. V. Viswanathan, Z. Nie, C. Wang, Y. Song, J.-G. Zhang, and J. Liu,Chem. Mater., 20, 3435 (2008); 45. Y. Bai, Z. M. Liu, N. Q. Zhang, andK. N. Sun, RSC Adv., 5, 21285 (2015), as determined from refinement ofthe relevant XRD profile. At the highest scan rate of 5.0 mV/s, thecathodic wave showed distortion, leading to a broadening of theappearance of the peak. The 5% physically sonicated LTO-MWNT sampledemonstrated A-Epeak values of 0.36, 0.44, 0.56, and 0.70 V at scanrates of 0.5, 1.0, 2.0, and 5.0 mV/s, respectively, as shown in FIG. 7b. The cathodic wave at the 5.0 mV/sec scan rate also gave rise todistortion, thereby leading to a broadening of the appearance of thepeak.

The CV for the 5% covalent LTO-MWNT sample (FIG. 6c ) depicts A-Epeakvalues which were 0.32, 0.44, 0.62, and 0.93 V at scan rates of 0.5,1.0, 2.0, and 5.0 mV/s, respectively. The A-Epeak values for thecovalently attached sample were comparable to the in situ and thesonicated samples measured at scan rates of 0.5 to 2.0 mV/s, yet thesewere higher at the scan rate of 5.0 mV/s. The CV peaks associated withthe covalently attached composites are wider as compared with the otherthree preparative analogues, believed to be the result of the largercharge transfer resistance imparted by the presence of the APTESlinkers.

By comparison, the LTO-MWNT composite prepared through π-π interactionsdisplayed A-Epeak values of 0.25, 0.35, 0.45, and 0.61 Vat scan rates of0.5, 1.0, 2.0, and 5.0 mV/s, respectively, as shown in FIG. 7d . Thesmallest A-Epeak values among all samples analyzed at each scan rate andtherefore, the fastest kinetic behavior observed are noted with thisLTO-MWNT composite, prepared with the MBA linker. It is also worthhighlighting that this sample exhibited the best consistency among allof the variously prepared LTO-MWNT composites tested.

For all of the prepared samples, the anodic and cathodic peak currentvalues were acquired and plotted versus the square root of the scanrate, as shown in FIGS. 8a and 8b , in order to determine (i) if thecharge transfer kinetics were fast enough to obey the Randles-Sevcikequation, A. J. Bard and L. R. Faulkner, Electrochemical Methods:Fundamentals and Applications, Wiley, New York (2000), and equallysignificantly, (ii) the Li-ion diffusion coefficients of all of thesamples analyzed. The sample prepared by the physical sonication methodyielded correlation coefficients of 0.99 and 0.96 for the cathodic andanodic peak currents, respectively. The in situ sample gave rise tocorrelation coefficients of 0.99 and 0.98 for the cathodic and anodicpeak currents, while the covalent sample highlighted correlationcoefficients of 0.99 and 0.96 for the cathodic and anodic peak currents,respectively. The sample prepared through π-π interactions generatedcoefficients of 0.99 for both the cathodic and anodic peak currents.These results indicate that the fastest kinetics among the fourattachment modalities were exhibited by the LTO MWNT sample generatedusing π-π interactions, a composite whose formation was mediated by thearomatic, conjugated 4-MBA linker.

Based upon the Randles-Sevcik equation shown below, the Li-ion diffusioncoefficient can be calculated from the slope of the graph, obtained byplotting i_(p) as a function of the square root of the scan rate.i _(p)=2.69*10⁵ n ^(3/2) AD ₀ ^(1/2) C* ₀ν^(1/2)

The Li-ion diffusion coefficients are found to be 3.4×10⁻⁸, 1.5×10⁻⁷,5.9×10⁻⁸, and 2.3×10⁻⁷ cm²/s, for the samples derived from physicalsonication, in-situ deposition, covalent attachment, and π-π:interactions, respectively. The sample prepared by π-π interactionsyielded a much higher diffusion coefficient as compared with the othersample types. Therefore, we can conclude that the diffusion coefficientand charge transfer are likely to be favorable for the π-π interactionsample.

Galvanostatic Charge-Discharge Cycling and Rate Capability—

The electrochemical cells were tested in galvanostatic mode for bothdischarge and charge processes. The LTO-MWNT composite samples weresubjected to a total of 105 discharge-charge cycles, including of threerounds of 35 cycles at discharge rates of C/2 (20 cycles), 20 C (5cycles), 50 C (5 cycles), and 100 C (5 cycles). This testing providedthe opportunity to assess both rate capability and capacity retention.Different strategies of integration with and attachment of MWNTs ontoLTO were investigated on samples possessing 5% MWNT loading amounts, asshown in FIGS. 9a -d.

The capacity retention for the four as-generated LTO-MWNT composites wasinvestigated. At cycles 20, 55, and 90, the capacities for the compositegenerated by the covalent method using the APTES linker were measured tobe 148,145, and 143 mAh/g, respectively, under a rate of C/2. Thecapacities for the composite synthesized by the in situ protocol werefound to be 150, 150, and 149 mAh/g, while the capacities for thecomposite derived from the physical sonication technique were computedto be 161, 160, and 159 mAh/g, respectively. Notably, the capacities forthe sample generated with the MBA linker were the highest of the samplestested; specifically, the data yielded values of 174, 171, and 170mAh/g. Thus, the LTO-MWNT sample prepared using the covalent methodexhibited a slightly poorer capacity retention of ˜97% from cycles 20-90as compared with the samples prepared via either physical sonication, insitu direct deposition, or π-π interaction methods, all of which yieldedapproximately 99% capacity retention from cycles 20-90.

At a rate of C/2, whereas the sample prepared by physical sonicationyielded a capacity of 161 mAh g-1, the analogous LTO-MWNT 5 wt %composite fabricated by the π-π interaction protocol out-performed allof the other materials tested. In fact, this π-π interaction derivedcomposite delivered a capacity of 175 mAh g-1, essentially achieving theexpected theoretical capacity predicted for the LTO material. Thedifferences among the measured capacities under the C/2 rate for theLTO-MWNT 5 wt % composites, synthesized by the in situ direct-depositionapproach as well as by covalent attachment protocols were minimal,yielding 150 and 148 mAh/g, respectively.

The rate capability of the material samples was also assessed, as can beseen from FIGS. 7a-d . Under C/2 conditions, both the in situ preparedLTO-MWNT sample and the sample prepared via physical sonicationdemonstrated an abrupt voltage drop upon initial discharge to ˜20 mAh/g,followed by a broad voltage plateau at ˜1.55 V, as depicted in FIGS. 9aand 9b , respectively. The sample generated by the π-π interactionutilizing the MBA linker displayed a slightly more gradual voltage dropupon initial discharge, followed by the appearance of a long plateau at1.55 V (FIG. 9d ). The sample prepared using the covalent attachmentwith the APTES linker gave rise to a more gradually sloping voltageprofile with higher voltages from 2.2-1.6 V out to 60 mAh/g, andfollowed by a plateau at approximately 1.55 V for the remainder of thedischarge process, as shown in FIG. 9c . Even though the realizedcapacities for the four electrode types were similar under low ratedischarge conditions, dissimilarities in the shapes of the voltageprofiles became rather more apparent and pronounced at higher rates.

Under high rates of discharge, more significant differences wereobserved, wherein the highest delivered capacity was achieved by thesamples prepared via the π-π interaction method. For example, under a 50C discharge rate, these latter π-π interaction derived samples delivered145 mAh/g, whereas the sonication-induced, in situ-derived, as well ascovalently-generated samples only produced 90, 71, and 16 mAh/g,respectively. Notably, the covalently derived samples with the APTESlinker had no functional capacity under a higher 100 C discharge rate,whereas the samples prepared by physical sonication, the in situdeposition method, and the π-π interaction protocols produced capacitiesof 15, 8, and 77 mAh/g at a similar 100 C discharge rate. Significantly,the sample derived from the π-π interaction protocol using the 4-MBAaromatic, conjugated linker gave rise to a capacity of 77 mAh/g with anextended plateau at about 1.16 V, thereby rendering it as the mostpromising high-rate LTO heterostructure.

Under cycling, the samples prepared via the physical sonication methoddelivered 161, 145, 90, and 15 mAh/g, under discharge rates of C/2, 20C, 50 C, and 100 C, at cycles 20, 25, 30, and 35, respectively, asdepicted in FIG. 10. FIG. 10 depicts discharge capacity versus cyclenumber for lithium/LTO-MWNT electrochemical cells created with activematerial composites prepared using sonication, covalent attachment, insitu deposition and π-π interactions, respectively at a 5% MWNT loadinglevel—the 3D flower LTO “control” sample tested in the same program alsois shown to effect comparison.

By contrast, the 5% in situ LTO-MWNT material resulted in capacities of150, 133, 71, and 8 mAh/g, while the sample prepared by the covalentmethod using the APTES linker gave rise to corresponding capacities of148, 105, 16, and 0.15 mAh/g, respectively, through cycles 20, 25, 30,and 35. It is worth noting that the ‘control’ sample, i.e. the pure 3Dflower LTO without any MWNT incorporation, yielded capacity values of153, 141, 109, and 24 mAh/g, respectively, through cycles 20, 25, 30,and 35. The figures highlight that the sample prepared by physicalsonication showed improvement at C/2 and 20 C discharge rates, but thesamples derived from both in situ and covalent attachment strategiesdisplayed lower capacity readings as compared with the pure LTO.

By contrast, the sample prepared from π-π interactions, using thearomatic, conjugated 4-MBA linker displayed outstanding performance bygiving rise to capacity values of 174, 163, 146 and 77 mAh/g for cycles20, 25, 30, and 35 respectively. By means of comparison with the‘control’ sample itself, the sample with the aromatic, conjugated MBAlinker delivered 37 mAh/g higher capacity than the pure, unmodified 3Dflower LTO sample at a 50 C discharge rate and 53 mAh/g higher capacityat a 100 C discharge rate.

In all cases, the trend in the charge capacities reflected thecorresponding changes in the discharge capacities measured under thesame rate. These quantifiable differences are consistent with priorobservations from the CV experiments. Indeed, (i) the high reversiblecapacity and the excellent cycling stability at C/2 coupled with (ii)the high discharge rates of 20 C and 50 C for the LTO-MWNT 5 wt %composites, generated by noncovalent π-π interactions using thearomatic, conjugated 4-MBA linker, are clearly superior to those ofpreviously reported values associated with LTO-carbon nanotube compositemotifs. A detailed comparison of our results in the context of theexisting literature is presented.

Electrochemical Impedance Spectroscopy (EIS) Data—

Electrochemical impedance spectroscopy was used to compare the inherentresistance of the Li/LTO-MWNT cells with 5% MWNT loading, prepared usingdifferent modes of attachment, as shown in FIG. 11. The data were fitusing an equivalent circuit, where R1 was attributed to ohmic resistanceand R2 was ascribed to charge transfer resistance. The R1 values wereconsistent for all four cell types, with a range of 1.5-2.3 ohms. The R2quantity, however, gave rise to more variation among the cells, withmeasured values of 52, 65, and 89 ohms noted for the cells prepared fromLTO-MWNT samples associated with sonication, in situ deposition, andcovalent attachment modalities, respectively.

The higher charge transfer resistance ascribed to the covalentattachment modality in particular was consistent with the poorer ratecapability discussed above in the context of the voltammetry andgalvanostatic testing results. Moreover, these results are consistentwith a study, wherein covalent attachment of Fe₃O₄ onto a glassy carbonelectrode surface using a 3-aminopropyltriethoxysilane (APTES) linkerled to the observation of a higher charge transfer resistance ascompared with a bare glassy carbon surface. H. S. Yin, Y. L. Zhou, T.Liu, T. T. Tang, S. Y. Ai, and L. S. Zhu, J. Solid State Electrochem.,16, 731 (2012).

To account for this behavior, we note that it has been reported that thepresence of unwieldy, sterically bulky, and non-conjugated ligands canfunctionally act as an undesirable potential barrier which can therebyinhibit the degree of charge transport between adjacent nanoparticlesand nanostructures. L. Wang, J. Han, B. Sundahl, S. Thornton, Y. Zhu, R.Zhou, C. Jaye, H. Liu, Z. Q. Li, G. T. Taylor, D. A. Fischer, J.Appenzeller, R. J. Harrison, and S. S. Wong, Nanoscale, 8, 15553 (2016);T. Virgili, I. S. Lopez, B. Vercelli, G. Angella, G. Zotti, J.Cabanillas-Gonzalez, D. Granados, L. Luer, R. Wannemacher, and F.Tassone, J. Phys. Chem. C, 116, 16259 (2012). An increased chargetransfer resistance associated with the covalent attachment processusing APTES would also account for not only the lower delivered capacitybut also, due to increased polarization effects, the poorer capacityretention observed.

By contrast, the sample generated from the π-π interaction method usingthe aromatic, conjugated 4-MBA linker displayed only 24 ohms as an R2value, indicating the most favorable charge transfer process occurringamong all of the LTO-MWNT composites tested. This finding moreovercorroborates previous published results which suggest thatelectron-rich, conjugated systems are more efficacious at enabling thecharge transfer process as compared with their non-conjugatedcounterparts. L. Wang, J. Han, B. Sundahl, S. Thornton, Y. Zhu, R. Zhou,C. Jaye, H. Liu, Z. Q. Li, G. T. Taylor, D. A. Fischer, J. Appenzeller,R. J. Harrison, and S. S. Wong, Nanoscale, 8, 15553 (2016).

The inventors have correlated the electrochemical performance of thesecomposite materials with their corresponding attachment chemistry. Forexample, in this study, the LTO sample with the 5% MWNT loading preparedvia the π-π interaction method evidenced the highest delivered dischargecapacity at every C rate from C/2 to 100C with the most notabledifferences apparent under discharge rates ≥20 C, due to a much lowercharge transfer resistance as compared with those of the other LTO-MWNTcomposites analyzed. It is worth further mentioning that these LTO-MWNTcomposites, produced by π-π interactions, exhibited a reproducibly highrate capability and a desirable cycling stability, i.e. delivering 174mAh g-1 at C/2 with a 99% capacity retention from cycles 20-90, 163 mAhg-1 at 20 C with a 97% capacity retention from cycles 25-95, and 146mAh/g at 50 C with a 90% capacity retention from cycles 30-100.

These values denote clearly superior performance to those of anypreviously reported LTO-carbon nanotube composite materials, to date,especially under these relatively low loading conditions. Notably, theLTO-MWNT samples prepared via the covalent attachment scheme delivered alower capacity and displayed 97% capacity retention from cycle 20 tocycle 90 at C/2 rate as compared with the higher capacity and 99%capacity retention for the set of physically sonicated, in situ, and π-πinteraction samples. The voltammetric and galvanostatic data coupledwith the impedance results indicate slower kinetics for the LTO-MWNTheterostructures, prepared using the covalent attachment approach,denoting data consistent with a prior report on a totally differentsystem wherein increased charge transfer resistance was found to havebeen associated with a covalent coupling protocol involving the3-aminopropyltriethoxysilane (APTES) linker. H. S. Yin, Y. L. Zhou, T.Liu, T. T. Tang, S. Y. Ai, and L. S. Zhu, J. Solid State Electrochem.,16, 731 (2012).

The inventive fabrication method's use of deliberative processingprotocols in order to tune and control fundamental anode materialproperties and realize truly favorable cycling performance coupled withthe high discharge capacity detected, highlighting distinctiveadvantages for the hierarchical architectures from a batteryperspective.

While the invention has been shown and described with reference tocertain embodiments of the present invention thereof, it will beunderstood by those skilled in the art that various changes in from anddetails may be made therein without departing from the spirit and scopeof the present invention and equivalents thereof.

What is claimed is:
 1. A method of fabricating nanocomposite anodematerial comprising lithium titanate (LTO)-multi-walled carbon nanotube(MWNT) composite structures intended for use in a lithium-ion battery,the method comprising: providing multi-walled carbon nanotubes (MWNTs)having surfaces, onto which functional oxygenated carboxylic acidmoieties are arranged; generating 3D flower-like, lithium titanate (LTO)microspheres having constituent nanosheets within the flower-likelithium titanate (LTO) microspheres; and anchoring theacid-functionalized MWNTs onto surfaces of the 3D LTO flower-like LTOmicrospheres by π-π interaction strategy to realize the nanocompositeanode material; wherein the 3D LTO flower-like microspheres are anchoredto the acid-functionalized MWNTs by π-π interaction by first dispersingthe 3D LTO flower-like microspheres in an ethanolic solution of4-mercaptobenzoic acid (4-MBA) linker molecules.
 2. The method of claim1, wherein generating the 3D flower-like, lithium titanate (LTO)microspheres includes synthesizing thin, saw-tooth shaped constituentnanosheets of lithium titanate (LTO) and subjecting the saw-tooth shapedconstituent nanosheets to a calcination process in air, at a temperaturerange of between 400 and 600 degrees Centigrade, for a limited timeperiod, to realize the nanocomposite anode material.
 3. The method ofclaim 2, wherein the limited time period is in a range of about 1 toabout 5 hours.
 4. The method of claim 3, wherein the calcination processin air, occurs at a temperature of about 500 degrees Centigrade, for alimited time period of about 3 hours.
 5. The method of claim 1, whereinthe ethanolic solution of 4-mercaptobenzoic acid (4-MBA) linkermolecules and 3D LTO flower-like microspheres is stirred to yield 4-MBAfunctionalized 3D LTO flower-like microspheres in which terminalcarboxylic acid groups of the ligand are bound onto Ti sites localizedon the 3D LTO flower-like microsphere surfaces by either a monodentateor bidentate coordination mode, to realize molecular-coated LTO product.6. The method of claim 5, further including isolating themolecular-coated LTO product by vacuum filtration to remove any unbound4-MBA linkers and then reacting the molecular-coated LTO product withoxidized MWNTs.
 7. The method of claim 6, wherein the reacting includessonicating the molecular-coated LTO product in a mixture of ethanol andDMSO solvents to stabilize 7C-7C interactions between phenyl ringswithin aromatic, conjugated linker molecules and an underlying networkcomprising the oxidized MWNTs.
 8. The method of claim 1, wherein theproviding includes acid-functionalizing the MWNTs, and dispersing theacid-functionalized MWNTs in dimethyl sulfoxide (DMSO) byultrasonication.
 9. The method of claim 1, wherein the 3D LTOmicrospheres comprise nanosheets that provide for shortened Li-iondiffusion distances and enhanced contact area with electrolyte, whenused in a Li-ion battery.
 10. The method of claim 9, wherein thenanosheets are saw-tooth shaped, which are formed using a hydrothermalprocess.
 11. The method of claim 1, wherein the MWNTs are fabricated bydispersing pristine MWTNs in a solution of HNO3 by sonication andrefluxing to remove any remnant catalysts and carbonaceous impuritiesand generate the functional, oxygenated carboxylic moieties ontonanotube surfaces of the MWTNs.
 12. The method of claim 11, wherein theHNO3 concentration is 70% by weight.